An image-receiving element and a light-sensitive element for forming transfer images by a silver salt diffusion process or a color diffusion transfer process, as well as a chemical reaction mechanism for image formation, are well known in the art.
In particular, an image-receiving element for a peel apart type diffusion transfer process is most generally used as a photographic film unit united with a light-sensitive sheet, that is, for a film unit composed of a light-sensitive sheet having on a support at least one light-sensitive silver halide emulsion layer, an image-receiving sheet having on another support at least an image-receiving layer, and between the sheets a pressure rupturable container containing a processing solution. In the film unit, after imagewise exposing the silver halide emulsion layer(s), the film unit is passed through a pair of juxtaposed pressure-applying rollers to rupture the container and to spread the processing solution between the sheets, whereby the development thereof is performed. As the result of the development, image-forming material(s) are imagewise diffused from the emulsion layer(s) or the dye-providing compound-(hereinafter referred to as a coloring material)-containing layer(s) associated therewith into an image-receiving layer formed on another support and fixed therein, to form, thereby, a transferred image. For obtaining desired positive prints or slide, the image-receiving sheet is separated from the light-sensitive sheet but in this case, if the separation of the image-receiving sheet is not performed smoothly or the processing solution remains on the image-receiving sheet, the images formed on the image-receiving sheet are stained, thus greatly reducing the image quality.
For easily separating the image-receiving sheet from the light-sensitive sheet and for preventing a processing solution from remaining on the image-receiving layer, it is known to form a layer referred to as a "stripping layer" on the image-receiving layer (at the processing solution spreading side).
Examples of materials for this kind of stripping layer are usually water-soluble or hydrophilic polymers such as gum arabic (U.S. Pat. Nos. 2,759,825; 4,009,031, etc.,); hydroxyethyl cellulose (U.S. Pat. No. 2,759,825 and Japanese Patent Application (OPI) No. 8237/72; the term "OPI" indicates an unexamined published patent application open to public inspection); methyl cellulose, ethyl cellulose, and nitrocellulose (U.S. Pat. No. 2,759,825); cellulose acetate hydrogen naphthalate (Canadian Pat. No. 681,777 and Japanese Patent Application (OPI) No. 41,623/72); carboxymethyl cellulose (U.K. Pat. No. 2,012,064); cellulose derivatives (Japanese Patent Publication No. 24,075/70; starch ethers (Japanese Patent Publication No. 35,820/75); galactomannan (U.K. Pat. No. 869,190); pectin (U.S. Pat. No. 2,759,825); phthalated gelatin (Japanese Patent Publication No. 24,075/70 and Japanese Patent Application (OPI) Nos. 74,431,79; 126,535/79); sodium alginate (U.S. Pat. No. 2,759,825); polyvinyl alcohol (U.S. Pat. No. 2,759,825 and U.K. Pat. No. 2,012,064, and Japanese Patent Publication No. 24,075/70); polymethacrylic acid (U.S. Pat. No. 2,759,825); etc. However, these materials have drawbacks in that the physical development center in the image-receiving layer (1) is denatured owing to the hygroscopicity of the material to change the density and color tone of the transferred images; (2) since natural materials are frequently used as the stripping layer materials described above, they sometimes deteriorate or get musty when they are stored as raw materials, i.e., they are unstable with the passage of time in having a tendency to be denatured; and (3) since water is used as a coating solvent for the materials, the energy required for drying is large and it takes a long period of time to dry the coated layer of the material; etc.
Furthermore, the use of the above-described materials is frequently accompanied by the problem that the transferred silver images show metallic luster, and the transferred density becomes low due to hindrance in the transfer.
For overcoming these problems, it has been proposed to use water-insoluble synthetic polymers such as a vinyl acetate-maleic anhydride copolymer and a methyl methacrylate-acrylic acid copolymer (described in Japanese Patent Publication No. 15,902/70); a barbituric acid-formalin condensation product (described in Japanese Patent Publication No. 4333/74); graft copolymers of gelatin sufficiently reacted with a dicarboxylic anhydride such as phthalic anhydride, etc., and a monomer such as a vinyl ester, vinyl ether, acrylic acid ester, etc., or a mixture thereof (described in Japanese Patent Application (OPI) No. 65,133/81), etc., as the materials for the stripping layers. However, it has now been clarified as a result of our detailed investigations that the use of the polymers described in the above-described patents for the stripping layers is accompanied by the following demerits. That is, these polymers have a poor film-forming property and are resistant to forming a thin uniform coating. As a result thereof, the images obtained by development in the image-receiving sheet using the polymer for the stripping layer are poor in uniformity and form undesirable image unevenness, which greatly reduces the commercial value of the photographic materials.
Also, as another method for overcoming the aforesaid difficulties, it has been proposed to use a stripping layer composed of a ternary polymer of styrene, acrylic acid (or methacrylic acid), and methyl acrylate (or methyl methacrylate) dispersed in a water-soluble polymer such as hydroxyethyl cellulose as described in U.S. Pat. No. 4,366,227. However, when photographic material having the image-receiving layer using the polymer for the stripping layer regardless of the water-soluble polymer or water-insoluble polymer is developed at temperatures lower than normal room temperature (about 20.degree. C), the component of the processing solution attached to the image-forming sheet causes so-called "clouding", i.e., is crystallized by drying to form white turbidity of images on the image surfaces.
Moreover, when the photographic material having the image-receiving sheet using the polymer for the stripping layer is developed at temperatures higher than normal room temperature, the processing solution partially remains on the surface of the image-receiving sheet.